It is important that an optical fiber used as an optical transmission line in an optical communications system be a single mode at a signal light wavelength and that the cutoff wavelength of a high order mode be shorter than the signal light wavelength. The cutoff wavelength of a high order mode is defined as a wavelength at which the attenuation due to bend loss of the high order mode is 19.37 dB. In ITU-T G.650.1, a bend reference method and a multimode reference method are indicated as a method of measuring the cutoff wavelength of a high order mode of an optical fiber.
L.-A. de Montmorillon et al., OTuL3,OFC 2009 (Literature 1) describes a bend-insensitive fiber which has a refractive-index distribution having a trench form in a cladding and in which light will not easily leak out even when it is bent with a small radius of curvature. It has been difficult to correctly measure a cutoff wavelength of such optical fiber by a conventional test method for cut-off wavelength.
R. Morgan et al., Opt. Left., Vol. 15 (1990), No. 17, 947 (Literature 2) indicates that a spectrum called “whispering gallery mode” (WGM) occurs when a part of light which has leaked from a core is reflected at an interface having a large refractive-index-difference, for example, between glass and coating layer or between air and coating layer, and causes interference with light propagating through the core.